One of the biggest problems caused from vehicles using fossil fuel, such as gasoline and diesel oil, is creation of air pollution. A technology of using a secondary battery, which can be charged and discharged, as a power source for vehicles has attracted considerable attention as one method of solving the above-mentioned problem. As a result, electric vehicles (EV), which are operated using only a battery, and hybrid electric vehicles (HEV), which jointly use a battery and a conventional engine, have been developed. Some of the electric vehicles and the hybrid electric vehicles are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). In recent years, however, the use of a lithium-ion secondary battery has been attempted.
High output and large capacity are needed for such a secondary battery to be used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). For this reason, a plurality of small-sized secondary batteries (unit cells) are connected in series or in parallel with each other so as to construct a battery module, and a plurality of battery modules are connected in parallel or in series with each other so as to construct a battery pack.
In such a high-output, large-capacity secondary battery, however, a large amount of heat is generated from the unit cells during the charge and the discharge of the unit cells. When the heat generated from the unit cells during the charge and the discharge of the unit cells is not effectively removed, heat is accumulated in the unit cells with the result that the unit cells are degraded. Consequently, it is necessary to provide a cooling system for vehicle battery packs, which are high-output, large-capacity secondary batteries.
Generally, a cooling system for vehicle battery packs is constructed in an air-cooling structure using air as a coolant. In the air-cooling structure, air outside or inside the vehicle is introduced to cool the battery pack, and is then discharged out of the vehicle. Consequently, various technologies to improve the efficiency of the cooling system has been developed.
For example, Korean Unexamined Patent Publication No. 2001-57628 discloses a cooling system for vehicle battery packs, which includes a heater and an evaporator for controlling the humidity and the temperature of air when the air outside or inside the vehicle is used as a coolant. Also, Korean Unexamined Patent Publication No. 2005-70726 discloses a cooling system for vehicle battery packs, wherein cooling fans are disposed at an intake port and an exhaust port, respectively, so as to improve the cooling efficiency. Furthermore, Korean Unexamined Patent Publication No. 2005-41323 discloses a cooling system for vehicle battery packs, wherein the area of an air flow channel is variable so as to improve the cooling efficiency.
The above-mentioned disclosures have advantages of improving the cooling efficiency of the battery pack. Since air, as a coolant, is supplied from the outside of the vehicle, however, it is necessary to provide additional devices for controlling the temperature and the humidity of the air with the result that the size of the cooling system is increased. Also, since the air having passed through the battery pack, which has been heated, is discharged out of the vehicle, the size of the cooling system is further increased.
As another example, Japanese Unexamined Patent Publication No. 2004-1683 discloses a cooling system for vehicle battery packs, wherein an intake duct for introducing air extends to the upper part of a cover sheet of a trunk located at the rear of a rear seat of a vehicle. Also, Japanese Unexamined Patent Publication No. 2005-71759 discloses a cooling system for vehicle battery packs, wherein an intake duct extends to a cabin of a vehicle. These disclosures have advantages in that air existing inside the vehicle, the temperature and the humidity of which are maintained at specific levels, respectively, is used, and therefore, additional devices, which are necessary for the above-mentioned Korean Unexamined Patent Publications, are not used.
However, when the intake duct extends to the cabin or a region adjacent to the cabin so as to use air in the cabin or in the region adjacent to the cabin, noise resulting from the operation of the cooling system is transmitted to the cabin. Furthermore, when a fire occurs in the vehicles due to various causes (including fire or explosion caused due to the abnormal operation of the battery pack), flames or toxic gas may flow backward to the cabin through the intake duct. Also, the air having passed through the battery pack, which has been heated, is discharged out of the vehicle. Consequently, the Japanese Unexamined Patent Publications also have the same problems as described above.
Meanwhile, the vehicle battery pack must be mounted in a restricted space in the vehicle, and therefore, it is preferable that the vehicle battery pack be constructed in a compact structure. The size of the vehicle battery pack is decided depending upon the size of the battery module, which is constructed by stacking unit cells one on another, and the size of the cooling system, which covers the outer surface of the battery pack. In connection with this case, the above-mentioned prior arts essentially include an intake duct for introducing air from the outside of the vehicle or the cabin into the battery pack and an exhaust duct for discharging the air having passed through the battery pack, which has been heated, out of the vehicle. These ducts have a predetermined size, and therefore, the ducts serve as a principal factor deciding the size of the battery pack based on the cooling system for the battery module with the same condition.